Combine the sentences using the correlative conjunctions ‘so...that’:
I was sleepy. I couldn’t concentrate.
a) I was sleepy so that I couldn’t concentrate.
b) I was that sleepy so I couldn’t concentrate.
c) So that I was sleepy I couldn't concentrate.
d) I was so sleepy that I couldn’t concentrate.

Hint: Correlative conjunctions are used in the way they are placed. The conjunction “so...that” shall be placed in the order that they are written. The conjunction “so” is used before the word “that” with a word/phrase between the two conjunctions. For example, the structure of a sentence is “She was so depressed that she did not eat”.

Complete answer:
 Now that we know about the sentence structure, we can easily identify the correct answer.
Option a is wrong because the conjunctions are not to be placed side by side which is the exact case in option a. Using “so that” together means “because”. The sentence in option a does not make any sense.
Option b is wrong because the word that has been used before the word so has been used. It makes the sentence colloquial and wrong.
Option c is wrong because once again, the words so and that have been used together.
Option d is the correct answer because it means that sleep was the reason the person could not concentrate. The words so and that have been used in the correct sequence with a word (sleepy) between them.
So, the correct answer is “Option d”.

Note:
Read the sentence to see if it makes sense. Sometimes, the sentence structure is correct but the sentence ends up not having any meaning. It is always important that there is a word or a phrase between the two conjunctions so keep a note of that.